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Abstract

Introduction

In cross-sectional studies, patients with rheumatoid arthritis (RA) have higher coronary
artery calcium (CAC) than controls. However, their rate of progression of CAC and
the predictors of CAC progression have heretofore remained unknown.

Methods

Incidence and progression of CAC were compared in 155 patients with RA and 835 control
participants. The association of demographic characteristics, traditional cardiovascular
risk factors, RA disease characteristics and selected inflammatory markers with incidence
and progression of CAC were evaluated.

Results

The incidence rate of newly detected CAC was 8.2/100 person-years in RA and 7.3/100
person-years in non-RA control subjects [IRR 1.1 (0.7-1.8)]. RA patients who developed
newly detectable CAC were older (59±7 vs. 55±6 years old, p=0.03), had higher triglyceride
levels (137±86 vs. 97±60 mg/dL, p=0.03), and higher systolic blood pressure (129±17
vs. 117±15 mm Hg, p=0.01) compared to those who did not develop incident CAC. Differences
in blood pressure and triglyceride levels remained significant after adjustment for
age (p<=0.05). RA patients with any CAC at baseline had a median rate of yearly progression
of 21 (7–62) compared to 21 (5–70) Agatston units in controls. No statistical differences
between RA progressors and RA non-progressors were observed for inflammatory markers
or for RA disease characteristics.

Conclusions

The incidence and progression of CAC did not differ between RA and non-RA participants.
In patients with RA, incident CAC was associated with older age, higher triglyceride
levels, and higher blood pressure, but not with inflammatory markers or RA disease
characteristics.

Introduction

Patients with rheumatoid arthritis (RA) die prematurely [1] and the leading cause of death is coronary artery disease [2-7].

High resolution computed tomography is a non-invasive technique that allows the identification
and quantification of coronary artery calcium (CAC). Prior studies have shown that
CAC scores correlate with the presence and extent of coronary plaque [8] and its presence is associated with a higher risk of coronary heart disease [9-12].

Our previous research showed that patients with RA have higher CAC scores than control
subjects [13,14]. These studies indicated that there was an association between higher CAC scores
with sedimentation rate, smoking and a composite score of disease activity and severity.
However, these associations were established in cross-sectional analyses; thus, they
did not allow assessment of temporal sequence, which would further support causality.

The rate and predictors of progression of coronary atherosclerosis in RA are unknown.
Therefore, we set out to test the hypotheses that: (1) subclinical coronary artery
atherosclerosis, as measured by CAC, progresses more rapidly in patients with RA than
in controls from a population based study; and (2) in RA subjects measures of disease
activity, disease damage, inflammation and traditional cardiovascular risk factors
are independent predictors of higher incident rates and progression of coronary artery
atherosclerosis.

Methods

This is a prospective study of patients with RA and control subjects from two cohorts
that followed similar protocols, the Evaluation of Subclinical Cardiovascular Disease
and Predictors of Events in Rheumatoid Arthritis (ESCAPE RA) cohort [14-16] and the Multi-Ethnic Study of Atherosclerosis (MESA) [17]. The study design is summarized in Figure 1.

Cohorts

The ESCAPE RA cohort was assembled to study the prevalence and progression of atherosclerosis
and to identify risk factors for prevalent and progressive subclinical cardiovascular
disease in patients with RA. Details of this cohort have been described before [14]. In brief, the cohort was assembled in the greater Baltimore area. Eligibility criteria
included fulfillment of the 1987 American College of Rheumatology classification criteria
for RA [18] and age 45 through 84 years. Major exclusion criteria were: (1) self-reported history
of physician-diagnosed myocardial infarction, heart failure, coronary artery revascularization,
peripheral arterial disease, implanted pacemaker or defibrillator devices and current
atrial fibrillation; (2) weight exceeding 300 pounds; and (3) computerized tomographic
(CT) scan of the chest within six months prior to study enrollment. A total of 197
patients were enrolled in this study from October 2004 through May 2006. Of these,
195 patients completed the initial evaluation, including assessment for the presence
of CAC at baseline. Further, 155 patients had a three-year follow-up visit including
a second CAC measurement. These 155 patients constituted the RA study sample for the
present analyses.

Control subjects were part of the MESA (Multi-Ethnic Study of Atherosclerosis), a
population-based cohort assembled in 2000 to 2002 to study the prevalence, risk factors
and progression of subclinical atherosclerosis. Details of the study design have been
published [17]. In brief, individuals were enrolled if they were 45 through 84 years of age and
did not report a past history of a physician-diagnosed cardiovascular event. Eight
hundred and thirty-five participants from the Baltimore Field Center who completed
a follow-up evaluation, had CT evaluation at baseline and follow-up and were not taking
any disease modifying anti-rheumatic drug, constituted the comparison group for these
analyses. The study was approved by the Johns Hopkins Hospital Institutional Review
Board and MESA, with all participants providing informed consent prior to enrollment.

Study outcomes

CAC was ascertained with the use of cardiac CT using a multidetector row computed
tomography (MDCT) system [19]. Scans were transmitted electronically to the MESA CT reading center where calcium
scores were quantified using the methods described by Agatston [20]. Scoring of scans was blinded to the group allocation (RA and control).

Patients with RA had their second CAC measurement after an average of 3.2 (range:
2.2 to 4.2) years. In MESA participants, the second CAC scan was obtained on 391 participants
of the MESA cohort during visit 2 and on 444 during visit 3. The mean follow-up time
to repeat scan in controls was 2.3 (range: 0.9 to 4.6) years.

Given that CAC scores are highly skewed and that about half of the participants had
a calcium score of zero at baseline, two pre-specified outcomes for subgroup analyses
were defined following the design previously described by Kronmal et al. [21]:

– Incidence of CAC: all participants with a CAC score of zero at baseline were included
in the analysis of incident CAC. The incident case definition required progression
to a positive calcium score (≥1) over time.

– Progression of CAC: restricted to those participants with any detectable CAC at
baseline.

Study covariates

Demographic and clinical characteristics, cardiovascular risk factors, serum/plasma
concentrations of inflammatory markers in RA patients and controls were collected
following similar protocols.

Clinical evaluation of cardiovascular risk factors

– Blood pressure (BP) was measured three times while individuals were sitting. The
average of the last two measurements was used in the analysis. Hypertension was defined
by systolic BP ≥140 mmHg, diastolic BP ≥90 mm Hg, or antihypertensive drug use.

– Diabetes was defined as a fasting glucose ≥126 mg/dL or use of anti-diabetic medications.

– Smoking status was ascertained by self-report.

Laboratory methods

Fasting sera and plasma were separated by centrifugation and stored at -70°C. Total
and high density lipoprotein (HDL) cholesterol, triglyceride, C-reactive protein (CRP),
IL-6 and fibrinogen were measured at the MESA core laboratory, the Laboratory for
Clinical Biochemistry Research (University of Vermont). In patients with RA, anti-cyclic
citrullinated peptide (anti-CCP) antibodies and rheumatoid factor (RF) were measured
as previously described [14]. Low density lipoprotein (LDL) cholesterol was calculated using the Friedewald equation
[19].

RA disease characteristics

The number of tender and swollen joints was ascertained and disease activity (DAS28)
calculated using the 28-joint count and CRP [22]. Radiographic damage was quantified by the Sharp score and functional capacity was
determined by the Health Assessment Questionnaire (HAQ) [23].

Anti-CCP antibody and RF greater than or equal to 60 and 40 units, respectively, met
the definition of seropositivity.

The presence of HLA associated RA susceptibility alleles (the so-called 'shared epitope’)
was defined as the presence of alleles QKRAA, QRRAA, QRAAA and RRRAA at positions
70 to 74 of Exon 2 of HLA-DRB1 using Allele SEQR HLA-DRB1 SBT kits (Abbott Molecular,
Inc., Des Plaines, IL, USA). Radiographs of the hands and feet were scored using the
Sharp-van der Heijde method [24] by a single, trained radiologist. As previously described, there were five subjects
with incomplete radiographic assessments, in whom the scores were imputed from the
available data [25].

Statistical methods

For all variables, means and standard deviations summarize normally distributed data.
For variables with skewed distribution, the data are presented as medians and interquartile
ranges (IQRs). Clinical and laboratory data ascertained longitudinally are expressed
as baseline and as average over follow-up. Categorical variables are presented as
proportions. Differences between continuous variables were tested using the Wilcoxon-rank
sum test or the t-test and between categorical variables, with the Chi-squared or
Fisher’s exact test.

The incidence rate of CAC in RA patients and control subjects is presented as person-years
and compared using Poisson regression. Multivariable regressions were modeled to examine
if any differences were independent of traditional cardiovascular risk factors, using
backward elimination by blocks.

Yearly progression rate was compared among RA patients and controls with CAC >0 at
baseline. Robust linear regressions were modeled to examine whether progression was
greater in RA patients than in controls and if any association was independent of
CAC score at baseline and traditional cardiovascular risk factors.

The associations between traditional risk factors and progression of CAC were explored
and heterogeneity was tested between 'caseness’ and selected variables with multiplicative
interaction models.

Finally, among RA patients, the association between traditional cardiovascular risk
factors and RA disease characteristics with progression of CAC was also explored.
Variables included disease activity, radiographic damage, medications and markers
of inflammation. Poisson regression or robust linear regression was used, as appropriate.

All statistical tests were calculated using a 5% two-sided significance level using
STATA/IC 11.0 (StataCorp, College Station, Texas, USA).

Results

Table 1 presents the clinical characteristics at baseline of the 155 RA patients and the
835 MESA participants. Patients with RA were, on average, four years younger than
control subjects. There were higher proportions of women, Caucasians and individuals
who completed a 12th grade education in the RA group than in the control group (each
P <0.05).

Patients and controls had a similar prevalence of hypertension, but diabetes at baseline
was more prevalent in controls than in RA patients. Average HDL cholesterol was higher
in RA participants than in controls, but there were no statistically significant differences
in average LDL cholesterol or triglyceride levels. Not surprisingly, patients with
RA had higher concentrations of IL-6, on average (P <0.001). However, RA patients did not have higher concentrations of CRP or fibrinogen,
on average, compared to controls.

Patients with RA had a median disease duration of nine years and a mean DAS28 of 3.7
± 1.1. Seventy percent had the shared epitope, 65% were seropositive for RF and 77%
seropositive for anti-CCP antibodies. The median (IQR) Sharp score was 55 (16 to 120)
and the median HAQ score was 0 (0 to 1).

Incident CAC

Table 2 presents the incidence rates of detectable CAC. Of the 73 patients with RA and a
CAC score of zero at baseline, 20 (27%) developed detectable CAC during an average
of 3.3 ± 0.3 years of follow-up, resulting in an incidence rate of 8.2 per 100 person-years.
In the control group, there were 381 participants free of CAC at baseline. Among those,
65 (17%) developed detectable CAC over an average of 2.4 ± 0.9 years of follow-up,
resulting in an incidence rate of 7.3 per 100 person-years.

Table 2.Incidence rate of CAC (among participants with no CAC at baseline)

When both incidence rates were compared, the unadjusted incidence rate ratio (IRR)
was 1.14 (95% CI 0.73 to 1.75). This result did not change substantially after adjustment
for demographic variables and traditional cardiovascular risk factors. A sensitivity
analysis, restricting controls with similar time to follow-up as RA patients, yielded
similar results (data not shown).

Progression of CAC

RA patients with any detectable CAC at baseline had a median yearly rate of progression
of 21 (7 to 62) Agatston units. Although the IQR varied slightly, the median rate
of progression was also 21 (5 to 70) Agatston units in control subjects. As shown
in Table 3, there was no statistically significant association between progression of calcium
scores and RA after adjustment for demographic variables and traditional cardiovascular
risk factors. Sensitivity analyses in which controls were restricted to those who
had similar time to follow-up as the RA patients, and another in which adjusted baseline
CAC scores were entered into the mode, gave us similar results (data not shown). In
a final sensitivity analysis in which RA patients (n = 86) were matched 1:1 to controls
(n = 86) for age, sex, race, diabetes, smoking, and dyslipidemia, progression rates
were not statistically different (19.3 (5.4 to 48.0) and 31.4 (4.2 to 66.5) Agatston
units for RA and controls, respectively (P = 0.5).

Table 3.Yearly progression among participants with any CAC at baseline

Risk factors associated with incident coronary artery calcium

RA patients who developed newly detectable CAC were older (59+/-7 versus 55 ± 6 years
old, P = 0.03), had higher concentrations of triglycerides at baseline (137 ± 86 versus
97 ± 60 mg/dl, P = 0.01) and higher systolic blood pressure (129 ± 17 versus 117 ± 15 mm Hg, P = 0.01) than those who did not develop any new coronary calcification. The interaction
analyses did not show disease status heterogeneity by age, triglycerides or systolic
blood pressure with regard to incident CAC (P values for interaction = 0.36, 0.25, and 0.11, respectively). The differences in
triglycerides and blood pressure remained statistically significant after adjustment
for age. There were no observed statistical differences in the concentration of inflammatory
markers or disease characteristics at baseline or as average over follow-up among
RA patients who developed coronary calcium and those who did not (Table 4).

Risk factors associated with progression of coronary artery calcium

Coronary calcium scores progressed at a higher rate in those patients with RA who
were Caucasian and had lower triglyceride concentrations (Table 5). This latter association became of borderline significance after adjustment for
age, sex, race, hypertension, diabetes and smoking (P = 0.08). No other associations were seen between patient characteristics and faster
rates of progression of CAC in patients with RA.

Table 5.Clinical Characteristics and the Risk of Progression of CAC (among those RA patients
with Detectable CAC at baseline)

Discussion

To the best of our knowledge, this is the first study evaluating determinants of CAC
incidence and progression in patients with RA. Our main findings can be summarized
in three parts. First, our results indicate that there were no statistically significant
differences in rates of incident CAC in patients and controls. Second, once patients
have any CAC, the progression is similar to the progression seen in controls. Third,
age, blood pressure and triglyceride concentrations, but not markers of inflammation
or measures of disease activity/damage, predicted newly identified coronary calcium
in RA.

Our analysis suggesting similar progression of CAC in patients and controls was not
concordant with our primary hypothesis or with a prior study that showed greater progression
of intima-media thickness (IMT) in the common carotid of RA patients compared to controls
[26]. There are several potential explanations for this apparent discordance. First, as
suggested by Maradit-Kremers et al., atherosclerosis may precede the clinical presentation of RA [27]. Thus, given that we focused on subclinical atherosclerosis, RA patients with prior
events, who may have contributed to an even greater progression of CAC, had been excluded,
and this could have introduced a differential bias. Second, it is likely that the
increased CV event rate in RA patients compared to controls in epidemiologic studies
is explained, at least in part, by rupture of vulnerable non-calcified plaque.

Many potential predictors of progression were explored in our analysis. Our exploratory
results indicate that age, hypertriglyceridemia and blood pressure were associated
with incident CAC but we did not find statistically significant associations with
RA-specific variables. In contrast, a recent analysis by our group indicated that
higher swollen joint counts and cumulative average CRP predicted progression of carotid
plaque in patients with RA [28]. While both carotid ultrasound (US) and CAC scores ascertain subclinical atherosclerosis,
CAC measures only calcified plaque while carotid US measures calcified and non-calcified
plaque as well as intima media thickness (IMT). Thus, each of these measures provides
different information. Calcified plaques are good predictors of myocardial infarction
(MI) and CVD mortality events in the general population, but they are more stable,
and thus, less prone to rupture and cause an event.

In contrast, carotid US measures both non-calcified and calcified plaques. Non-calcified
plaques account for approximately three-quarters of all coronary lesions [29], are associated with inflammation [30] and are also important predictors of cardiovascular hard events [31]. Thus, it is possible that the assessment of this type of plaque could improve cardiovascular
risk stratification in patients with RA. This study has several strengths. First,
it compares two contemporary cohorts and data were collected prospectively. Second,
both groups were studied following similar and rigorous protocols. Third, it uses
state of the art techniques to ascertain CAC and to measure inflammatory markers.

Our study also has some limitations. First, although the rate of successful follow-up
was high, 20% of patients with RA either declined or could not be contacted for their
final follow-up assessment. We hypothesized that patients who were lost to follow-up
may have had more severe disease; however, median baseline DAS and HAQ scores were
similar among patients who came back for a second visit and those who were lost to
follow-up. Second, the average follow-up was only 3.2 years in RA patients and 2.3
years in controls and, to account for this, required estimation of average yearly
progression. Longer follow-up and the evaluation of CV events, such as acute MI and
CV deaths, will be more informative. Third, type II error might explain lack of statistical
significance in some of the subgroup analyses. For example, a pre-study power analysis
estimated that to show a difference in CAC scores among individuals with any coronary
calcification detected at baseline would have required a difference of 26 Agatston
units per year between RA patients and controls. In addition, a younger cohort of
patients with lower burden of coronary atherosclerosis at baseline and higher RA disease
activity could have increased the likelihood of a positive result. Fourth, the lack
of association between RA and progression of subclinical coronary atherosclerosis
does not exclude the possibility of differences in progression of arterial calcification
in other vascular beds, such as thoracic aorta.

Conclusions

In summary, the incidence and progression of coronary calcium did not differ significantly
between RA and non-RA groups. In patients with RA, higher systolic blood pressure,
higher triglycerides concentrations and older age were significant predictors of incident
CAC over the period of follow-up, while inflammatory and RA disease characteristics
were not. Among patients with RA and CAC at baseline, no association was found between
traditional risk factors and CAC progression.

Competing interests

The authors declare that they have no competing interests.

Authors’ contributions

CPC participated in analysis and interpretation of the data and drafting and revising
the manuscript. JTG participated in design of the study, acquisition of the clinical
data, analysis and interpretation of the data and reviewing and revising the manuscript.
RK participated in the statistical design, analysis and interpretation of the data
and reviewing and revising the manuscript. WSP participated in design of the study,
acquisition of the carotid ultrasound data and reviewing and revising the manuscript.
ACG participated in the analysis and interpretation of the data and reviewing and
revising the manuscript. MP participated in the analysis and interpretation of the
data and reviewing and revising the manuscript. MS participated in the design of the
study, analysis and interpretation of the data and reviewing and revising the manuscript.
RD participated in acquisition of the coronary artery calcium data, reviewing and
revising the manuscript and gave final approval of the data. MJB participated in acquisition
of the coronary artery calcium data, reviewing and revising the manuscript and gave
final approval of the data. RSB participated in design of the study, acquisition of
the coronary calcium data and reviewing and revising the manuscript. PO participated
in the analysis and interpretation of the data and reviewing and revising the manuscript.
DB participated in acquisition of the coronary calcium data and reviewing and revising
the manuscript. JMB designed the study, participated in acquisition of all aspects
of data, participated in analysis and interpretation of data and drafted and revised
the manuscript. All authors read and approved the final manuscript.

Acknowledgements

We thank the ESCAPE RA staff, Marilyn Towns, Michelle Jones, Patricia Jones, Marissa
Hildebrandt, and Shawn Franckowiak, and the staff of the Johns Hopkins Bayview Medical
Center General Clinical Research Center and the field center of the Baltimore MESA
cohort and the MESA Coordinating Center at the University of Washington, Seattle for
their efforts.

This study was supported by Grant Numbers AR050026-01 (JMB) and 1K23AR054112-01 (JTG)
from the National Institutes of Health, National Institute of Arthritis and Musculoskeletal
and Skin Diseases; a Clinical Investigator Fellowship Award from the Research and
Education Foundation of the American College of Rheumatology (JTG); and the Johns
Hopkins Bayview Medical Center General Clinical Research Center (Grant Number M01RR02719).
Funding for this research was also made possible by the American College of Rheumatology
Research and Education Foundation’s Within Our Reach: Finding a Cure for Rheumatoid
Arthritis campaign (JMB) and the ACR/REF Ephraim P. Engleman Endowed Resident Research
Preceptorship (CPC). MESA is funded by contracts N01-HC-95159 through N01-HC-95166
and N01-HC-95169 from the National Heart, Lung, and Blood Institute.

References

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